One way that information may be communicated between two devices is by modulating light at one device to form an optical signal and transmitting the optical signal to the other device. One form of modulation that can be used in such optical communication is known as pulse-amplitude modulation (PAM), which, in the context of optical communication, involves modulating the power (intensity) of a light signal so as to encode data for transmission.
PAM-4 is a form of PAM in which, in the context of optical communication, there are four different power levels to which the optical signal may be modulated, each corresponding to a different transmission symbol. Each transmission symbol in a PAM-4 optical signal encodes a sequence of two binary bits, such as 00, 01, 10, and 11.